(i) Field of the Invention
The present invention relates to a reversible rolling method in which an optimum rolling pass schedule is rationally determined with respect to a reversible rolling system for mainly rolling a steel strip and rolling is performed, and a reversible rolling system for performing the rolling on the basis of the rolling pass schedule.
(ii) Description of the Related Art
In a reversible rolling system, provided with one rolling mill (a pair of combined rolling mills as occasion demands), for repeatedly reciprocating and passing one steel strip forward and backward in the rolling mill to advance the rolling at every pass, it is necessary to determine a draft schedule is set which determines a draft amount for each pass until a target plate thickness is finally obtained, and a rolling pass schedule for satisfying various conditions on the basis of the draft schedule.
As a method of determining the pass schedule of the reversible rolling system, techniques have heretofore been disclosed, for example, in JP-A-6-262225, JP-A-7-232205 and the like. An object of these inventions is to provide a method of setting the pass schedule to maximize the production amount.
The basic idea of the invention disclosed in JP-A-6-262225 comprises: preparing a draft amount table beforehand; and setting the rolling speed of each pass on the basis of the table so as to reach the upper limit of the thermal overload of a motor or a power supply.
The basic idea of the invention disclosed in JP-A-7-232205 comprises:
1. setting the draft ratio of each pass to be maximum on the basis of restrictive draft conditions such as a load, and setting the rolling speed at the maximum speed which is allowable from restrictive speed conditions such as a power; or PA1 2. determining the maximum speed on the basis of the restrictive conditions, and setting the draft ratio of each pass under the conditions. PA1 Therefore, by setting the value of the product Q to be substantially equal in each pass, the function A takes a value which becomes smaller as the pass becomes later by the action of the function B which becomes larger as the pass becomes later. Moreover, since the function A is in a relation of monotonic increase with respect to each pass draft amount, the draft amount is necessarily reduced as the pass becomes later. This corresponds to the draft schedule which follows the general idea in performing the reversible rolling. Additionally, also in the present invention, the functions A and B are selected so that the product Q forms the function substantially independent of the rolling speed as a result. Therefore, each pass rolling speed and the draft schedule can freely be determined independently of each other, and the rolling pass schedule can be determined to easily and rationally perform the reversible rolling without depending on the experiences. Moreover, in the present invention, the draft schedule can be determined using the actual rolling parameters (e.g., the average rolling power consumption, the average overload ration, and the pass interval time).
Moreover, a method of determining the pass schedule for the purpose of obtaining a high plate thickness precision in the reversible rolling system is also proposed in JP-A-51-72951. The basic idea comprises: dividing and storing the actually detected data of a plate thickness into two groups comprising the data during the rolling in a forward path and the data during the rolling in a backward path; obtaining estimated values independently of each other on the basis of the stored data in the respective groups; and gradually eliminating a difference from a theoretical value to perform adapted correction.
The invention of JP-6-262225 has a problem that the draft amount table is first required, but the method of preparing the table is not disclosed, and the table needs to be prepared by knowledge on the basis of experiences.
In this respect, in JP-A-7-232205, the method of preparing the draft schedule is proposed, and one effective method is proposed in setting the draft schedule for obtaining the maximum production amount. In the schedule set as described above, the rolling is regulated by various restrictions to be ultimately limited rolling, and it becomes extremely difficult to perform an actual rolling. Particularly, the restrictions on the system such as rolling load and torque become problems upon the rolling of the maximum plate width. Conversely, upon the rolling with the minimum plate width, most of the restrictions are determined, for example, by the shape difficult to be restricted. Moreover, the rolling for obtaining the maximum production amount of the system is not necessarily required for all the rollings, and the above-described rolling is usually a little. Particularly, in a hot strip mill which is a thin plate rolling system for use in a hot processing, it is usual to perform a reversible rolling in a coarse rolling process and subsequently perform a finishing rolling in a tandem rolling mill. In this reversible coarse rolling mill, the rolling for obtaining the maximum production amount of the system is not necessarily required, and the rolling may be performed in a rolling time which is approximately the same as a finishing rolling time. When the rolling is performed with an allowance with respect to system ability, the method of setting the pass schedule according to the latter invention can be said to be inappropriate.
Moreover, in the invention disclosed in JP-A-51-72951, when there is a little change in the numerous restrictive conditions, the pass frequency is increased from the time of the change to gradually perform the adapted correction between the theoretical and estimated values. This method has a disadvantage that the change of the restrictive conditions cannot quickly be handled, and the rolling including errors is repeated many times.